1. Field of the Invention
The present invention relates to a motor coil structure and more particularly to means for improving a space factor in a coil winding.
2. Description of the Prior Art
FIGS. 9 to 11 show a motor coil structure according to the prior art. For simplicity, the stator core 10 and rotor 16 are linearly drawn in these figures, but they are actually of cylindrical configuration.
The motor is a three-phase motor of a structure including three coils 12-U, 12-V and 12-W fixedly mounted on the stator core 10. To fix these coils 12-U, 12-V and 12-W, the stator core 10 is formed with a predetermined number of slots 14-U, 14-V and 14-W. Each of the coils 12-U, 12-V and 12-W is formed by a plurality of bound conductive wires. Each coil 12-U, 12-V or 12-W is wound around the core through the corresponding one of the slots 14-U, 14-V and 14-W as shown in FIGS. 10 and 11 and the end face of the stator core 10 is exposed as shown in FIG. 9. The portion of each coil 12-U, 12-V or 12-W wound around the end of the stator core 10 and spanned between the slots 14-U, 14-V, and 14-W is known as an interslot connecting wire.
As a three-phase current is supplied through the coils 12-U, 12-V and 12-W, a three-phase alternating magnetic field is produced. With an inner rotor structure, a rotor 16 is disposed within a cylinder-shaped space which is formed inside of the stator core 10. However, an outer rotor structure provides a rotor 16 disposed outside of the stator core 10. The rotor 16 includes permanent magnets 18 mounted thereon opposite to the stator core 10. When the three-phase alternating magnetic field is produced by exciting the coils 12-U, 12-V and 12-W, a torque is created in the rotor 16 to rotate the rotor 16.
In general, the size of the coil structure is evaluated by a space factor. The space factor is defined as a rate of occupying a cross-section of a corresponding slot 14.
On the other hand, in the prior art described above, the coils 12 are wound around the stator core 10 after the coils 12 are tied up in bundles. Although this manufacturing process makes the handling of the coils 12 easy and thus the manufacturing cost cheap, the process provides a coil structure with a space factor equal to or less than 50%. In other words, the coil structure and its manufacturing process have a problem in that the space factor is small.
In addition, because the interslot connecting wires are parts of the coils 12, the interslot connecting wires become large when the above-described process is used. As a result, the size of the stator core 10 along an axis of the motor becomes large.
To solve these problems, the making of the coils 12 by winding the wires around the stator core 10 without lying them up in bundles is preferable. However, when this process is used, not only are the manual operations to wind each wire around the stator core necessary and thus the manufacturing cost is increased, but also the inductance of each coil 12 becomes incorrect and the quality of the coil structure therefore varies.